Control arrangement



Nov. 1, 1966 G. KLIPPING ETAL 3,282,063

CONTROL ARRANGEMENT Filed Feb. 24, 1965 5 Sheets-Sheet 1 /8 TANK 1 0L VACUU M PUMP Fig.1

INVENTQRS Gustav Klipp Dieter vetterkinqa By Georg Wolentowatz ATTORNEYS Nov. 1, 1966 G. KLIPPING ETAL.

CONTROL ARRANGEMENT 5 Sheets-Sheet 2 Filed Feb. 24, 1965 m fl mmmw pl 5&3 526.. m mm ||I|IJ mwmmm 5 a a VW t GM 6 D Nov. 1, 1966 e. KLIPPING ETAL CONTROL ARRANGEMENT 5 Sheets-Sheet 5 Filed Feb. 24, 1965 INVENTORS Gustav Klipping Dieter Vetterkind 8: Georg \Ncllentowitz .ZDOEO wwnzmm QMTZ ESI BY (g %EY United States Patent 3,282,063 (IGNTROL ARRANGEMENT Gustav Klipping, Berlin, Dieter Vetterkind, Dusseldorf, and Georg Waleutowitz, Berlin, Germany, assignors to Max-Planck-Gesellschaft, Gottingen, Germany Filed Feb. 24, 1965, Ser. No. 434,853 Claims priority, application Germany, Feb. 25, 1964,

11 Claims. for. 62-45) The present invention relates to an arrangement for controlling the temperature of evaporators for refrigerants having low boiling points, such as helium, in which the refrigerant is fed into the evaporator by a vacuum pump at a rate dependent on the temperature at a selected point within the evaporator body.

Evaporators operating with liquids having low boiling points, such as helium, are especially suited for the continuous low temperature cooling of test samples or condensation surfaces. See G. Klipping, Ein Heliumverdampfer zur Erzeugung tiefer Temperaturen (A Helium Evaporator for Producing Low Temperatures), Kaltetechnik, vol. 13 (1961), pp. 250-255. In this type of evaporator, a supply pipe immersed in the liquid refrigerant communicates with the :hollow evaporator body. The evaporator body is connected to a vacuum pump by a feed pipe in which a refrigerant metering throttle is located. This throttle regulates the amount of refrigerant drawn into the evaporator by the vacuum pump in dependence to the temperature at a point within the evaporator body. Up to now, vapor pressure-operated bellows valves have been used as refrigerant metering devices. These devices have been regulated by control elements located within the evaporator body and filled with a suitable medium. Satisfactory temperature regulation and reliability have been obtained, even for extended periods of operation, using such devices. However, only temperatures within certain ranges can be achieved in this manner. These include temperatures below 42 Kelvin, between 13.8 and 27 Kelvin, between 58 and 112 Kelvin, and above 138 Kelvin. No temperature which is not within the ranges specified is achievable, since there are no refrigerants boiling under normal or reduced pressure outside these ranges. Any temperature within the above-mentioned ranges can be set without interruption of operation. It is, however, necessary to interrupt the operation when changing from one range to another, because the medium in the control element for the metering valve has to be changed. It is possible to bridge the gaps between the above-mentioned temperature ranges, for example, by heating the sample to be measured electrically, or by taking advantage of the natural temperature rise during the performance of the experiment. When using these methods, the duration of the experiment at a temperature between the ranges is necessarily limited. In many cases, a longer duration at some temperature between the. attainable ranges is required. Continuous temperature variation from the lowest temperature obtainable up to room temperature, without interruptions for changing the medium in the control element, is also often required.

It is therefore an object of the present invention to provide an arrangement for controlling the temperature of an evaporator for liquid refrigerants having low boiling points, which overcomes the above-mentioned defects of the prior art.

It is a futher object of the present invention to provide an arrangement for controlling the temperature of an evaporator for liquid refrigerants having low boiling points, wherein the refrigerant flows through a valve which cyclically opens and closes, in which the ratio of the time during which the valve is open to that during which it is closed is regulated according to the evaporator temperature.

The objects, as well as others, are achieved according to the invention, in which an electromagnetic valve is cyclically operated to control the flow of a refrigerant, and the transmission ratio of the valve is changed according to the temperature within the evaporator body, so that when the temperature rises, the transmission ratio increases, and when the temperature falls, the ratio decreases. The transmission ratio may be defined as the ratio between the time spent by the electromagnetic valve in its open position and the time spent by the valve in its closed position, during each period of its cyclic operation.

The electromagnetic valve can be controlled by a variable pulse voltage. The width of these voltage pulses is then varied proportionally (preferably linearly) to the change in resistance of a temperature sensing device, which in this case would be a temperature-dependent resistance element.

Additional objects and advantages of the present invention will become apparent upon consideration of the following description when taken in conjunction with the accompanying drawing in which:

FIGURE 1 shows schematically a refrigerant system and a block circuit diagram of the electrical temperature control arrangement.

FIGURE 2 is a circuit diagram of the electrical temperature control arrangement with a bridge supplied with a DC. voltage.

FIGURE 3 is a circuit diagram of the electrical tem perature control arrangement with a bridge supplied with an A.C. voltage.

Referring now to the drawings, FIGURE 1 shows a storage tank 1 for the liquid refrigerant, which discharges into the evaporator 3 via a supply pipe 2. A feed pipe 4 leads from the evaporator to vacuum pump 7. An electromagnetic value 5 and a bypass valve 6 are in parallel in the feed pipe 4. The bypass valve 6 is set to supply an amount of refrigerant that is too small to reach the wanted temperature at the evaporator. The additional amount of refrigerant supplied via the electromagnetic valve operating in dependence to the temperature changes at the evaporator causes the final cooling to the wanted temperature value. The vacuum pump is connected to a gas container (not shown) by means of a flange 8. The block diagrarnof the electrical temperature control system shows an electrical sensing device 9, in thermal communication with evaporator 3. This sensing unit controls the operation of the electromagnetic valve 5 through control units I and II. Changes in resistance of the electrical sensing device, caused by temperature changes in the evaporator 3, are transformed into voltage changes in control unit I. In control unit II, these voltage changes are transformed into changes in the pulse width of a pulse voltage generated in unit II. As shown in the schematic of FIGURE 2, the electrical sensing unit 9 is part of a conventional bridge circuit 10, which is supplied with DC. voltage by the power supply 11. The bridge may be initially balanced by means of a variable resistor 10a. The bridge 10 is connected to a pulse width modulator 13 through an amplifier 12, which has an output level control 12a. In another modification shown in FIGURE 3, the electrical sensing unit 9 is part of a conventional bridge circuit 16, which is supplied with A.C. voltage. Here too, the bridge may be balanced initially by means of a variable resistor 16a. The bridge 16 is connected to a pulse width modulator 13 through a multistage amplifier 17 with adjustable amplification rate and a phase discriminator 18.

According to the invention, a sum and difference amplifier, or differential amplifier, is used as pulse width modulator 13. It is supplied with a pulse voltage at one input (13a), generated by saw-tooth generator 15, and with a modulating voltage from amplifier 12 at the other input (13b). Amplifier 12 provides an output voltage proportional to the change in resistance of resistor 9 from its value at the desired temperature. The output of pulse width modulator 13 is connected to the electromagnetic control valve 14. -In the modification shown in FIGURE 3, the output voltage proportional to the change in resistance of resistor 9 is provided by multi-stage amplifier 17 in connection with phase discriminator 18.

The saw-tooth generator 15 includes resistors R1, R2, R3 and R4, as well as transistor T and a capacitor C. A four-layer diode D functions to discharge the capacitor C. The resistor R3 limits the maximum current flowing through four-layer diode D. By changing the value of only one element of the generator (resistor R4) the output frequency can be changed.

The input amplitude of the signal supplied to pulse width modulator 13 by saw-tooth generator 15 is substantially larger than the linear operation region of the amplifier. This strong overmodulation causes limiting in the output voltage to the amplitude of the linear amplifying region. The output voltage is consequently nearly rectangular. The transmission ratio of this rectangular voltage (defined as the ratio of the pulse width to the remainder of the period of the waveform) can be varied by shifting the point at which each pulse starts. This is accomplished by changing the modulating voltage, and in this case is regulated by the temperature in the evaporator, which governs the resistance of resistor 9 and thus the value of the bridge output.

In the preferred embodiment just described, the temperature sensing device is connected in a bridge circuit supplied with DC. voltage or supplied with an A.C. voltage. The control voltage for changing the transmission ratio of the electromagnetic valve is thus derived from the bridge null voltage. The Width of the voltage pulses can be varied in linear proportion to the change in the bridge null voltage. As has been shown, for actuating the electromagnetic valve, it is advantageous to use a saw-tooth voltage that is modulated according to the temperature at the electrical sensing device. As has further been demonstrated, a differential amplifier may be provided as a pulse width modulator, to vary the duration of the voltage pulses supplied by the saw-tooth generator. The saw-tooth voltage is fed to one amplifier input (13a) and the control voltage, corresponding to the temperature at the electrical sensing device, is fed to the other amplifier input (13b). The differential amplifier supplies a current which operates the electromagnetic valve. The input saw-tooth amplitude of the differential amplifier is considerably above its linear operating region. A fourlayer diode can be connected in parallel with the charging capacitor in the saw-tooth generator to control discharge thereof. The resistance changes of the electrical sensing device, caused by temperature changes which occur in the thermal vicinity thereof, are registered in the bridge circuit, which is supplied with an A.C. or (DC) voltage. Detuning of the bridge gives rise to a null voltage that is nearly linearly proportional to resistance change and which may be amplified in a conventional multistage amplifier having an adjustable amplification rate and a phase discriminator (in a conventional adjustable amplifier). This direct voltage is fed to a pulse width modulator, where it is used to vary the width of a voltage pulse that supplies the electromagnetically operated throttle valve in a linear manner. As has been shown, the voltage pulses are supplied by a generator 15, the capacitor C of which is discharged through a fourlayer diode D. The output frequency of generator 15 is variable, and should be set to the desired frequency of operation of the valve. A strongly overmodulated differential amplifier is used as the pulse width modulator.

Its output is a rectangular voltage that is obtained by switching the direct voltage applied to it according to the transmission ratio. This rectangular voltage is fed to the magnetic valve via a conventional output stage.

With this circuit arrangement, the transmission ratio of the electromagnetic valve is increased when the temperature at the sensing device rises, providing a larger amount of refrigerant, and is decreased when the temperature falls, reducing the supply of refrigerant.

It will be apparent that an advantage of this method over conventional ones lies especially in the fact that this temperature control arrangement can be set to maintain any temperature between the lowest reachable value (liquid helium boiling under reduced pressure) and room temperature, and that the temperature so set can be kept constant for long periods of operation. This means that temperature ranges that were ony accessible under certain undesirable conditions when using the vapor-pressure controlled valve are now fully accessible, without any disturbing interruption in operation. The temperature stability obtainable is better than that possible with the vapor-pressure temperature control. It is furthermore of advantage that this kind of temperature control does not depend on the outside pressure. It is also possible to obtain optimum adjustment of the desired temperature by varying the amplification.

The saw-tooth generator disclosed is especially suited for generating pulses for driving the valve; its circuit design is much simpler than that of conventional generators. Furthermore, the ratio of supply voltage to saw-tooth voltage is almost unity, and the flyback period is extremely short. By using the differential amplifier as a pulse Width modulator, the advantages of such an amplifier (for example independence of the output from interference caused by fluctuation of the supply voltage) are fully exploited. The proposed circuit arrangement provides good decoupling of the saw-tooth and modulating voltages, and the output is considerably amplified, which means that the circuit arrangement is much less subject to interference. The simplicity of the circuit arrangement and of its elements, the saw-tooth generator and the pulse width modulator, leads to excellent reliability and operating security of the control apparatus.

It will be understood that the above description of the present invention is susceptible to various modifications, changes and adaptations, and the same are intended to be comprehended within the meaning and range of equivalents of the appended claims.

What is claimed is: 1. An arrangement for controlling the temperature of an evaporator for liquid refrigerants having low boiling points, such as helium, wherein the refrigerant is supplied to the evaporator by means of a vacuum pump at a rate proportional to the temperature at a point within the evaporator body, said arrangement comprising, in combination:

a hollow evaporator body; means for feeding refrigerant to said body; periodically operable electromagnetic valve means for controlling the amount of such refrigerant entering said evaporator body through said feeding means;

temperature sensing means for sensing changes in temperature in said evaporator body; and

control means responsive to said sensing means for varying the transmission ratio of said valve as a function of the temperature changes sensed by said sensing means to increase the transmission ratio in response to a temperature rise in said body and to decrease the transmission ratio in response to a temperature drop in said body.

2. An arrangement as defined in claim 1, wherein said control means includes means for supplying a train of variable-width voltage pulses to said electromagnetic valve for Opening and closing said valve in accordance with the width of sa d pulses.

3. An arrangement as defined in claim 2, wherein said sensing means is a temperature dependent resistance element, and said control means includes means for modulating the width of said voltage pulses to be proportional to the change in resistance from a predetermined value of said temperature dependent resistance element.

4'. A circuit arrangement as defined in claim 3, said control means including a bridge circuit, one arm of which is said electrical sensing device; means for supplying said bridge circuit with a DC. voltage; and circuit means for supplying the bridge null voltage to said pulse width modulating means.

5. A circuit arrangement as defined in claim 3, said control means including a bridge circuit, one arm of which is said electrical sensing device; means for supplying said bridge with an AC. voltage; and circuit means for supplying the bridge null voltage to said pulse width modulating means.

6. A circuit arrangement as defined in claim 4, wherein said pulse width modulating means varies the width of said voltage pulses in linear proportion to the bridge null voltage.

'7. A circuit arrangement as defined in claim 5, wherein said pulse width modulating means varies the width of said voltage pulses in linear proportion to the bridge null voltage.

8. A circuit arrangement as defined in claim 2, wherein said control means includes saw-tooth voltage generating means, and means responsive to said sensing means and to said saw-tooth voltage for generating voltage pulses which vary in width according to the temperature of said sensing means.

9. A circuit arrangement as defined in claim 8, wherein said modulating means is a differential amplifier having first and second inputs, said saw-tooth voltage generating means having an amplitude substantially greater than the linear operating region of said dilTe-rential amplifier and said saw-tooth voltage generating means being connected to said first input; means for providing a control voltage corresponding to the temperature at'said electrical sensing device, said control voltage means being connected to said second input; and circuit means connecting the elect-romagnetic valve means with the output of said diiferential amplifier. 5 10. A circuit arrangement as defined in claim 8, wherein said saw-tooth voltage generating means includes an output capacitor connected in parallel with a four-layer diode for controlling discharge of said capacitor.

11. An arrangement for controlling the temperature of an evaporator for liquid refrigerants having low boiling points, wherein the refrigerant is supplied to the evaporator by means of a vacuum pump at a rate proportional to the temperature at a selected point within the evaporator body, said arrangement comprising, in combination:

a hollow evaporator body;

means for feeding refrigerant to said body;

periodic electromagnetic valve means operable in closed and open positions for controlling the amount of refrigerant entering said evaporator body through said feeding means, said valve means having a transmission ratio equal to the time spent per period in its open position to the time spent per period in its closed position;

temperature sensing means for sensing changes in temperature in said evaporator body; and

control means responsive to said sensing means for varying the transmission ratio of said valve as a function of the temperature changes sensed by said sensing means to increase the transmission ratio in response to a temperature rise in said body and to decrease the transmission ratio in response to a temperature drop in said body. 

1. AN ARRANGEMENT FOR CONTROLLING THE TEMPERATURE OF AN EVAPORATOR FOR LIQUID REFRIGERANTS HAVING LOW BOILING POINTS, SUCH AS HELIUM, WHEREIN THE REFRIGERANT IS SUPPLIED TO THE EVAPORATOR BY MEANS OF A VACUUM PUMP AT A RATE PROPORTIONAL TO THE TEMPERATURE AT A POINT WITHIN THE EVAPORATOR BODY, SAID ARRANGEMENT COMPRISING, IN COMBINATION: A HOLLOW EVAPORATOR BODY; MEANS FOR FEEDING REFRIGERANT TO SAID BODY; PERIODICALLY OPERABLE ELECTROMAGNETIC VALVE MEANS FOR CONTROLLING THE AMOUNT OF SUCH REFRIGERANT ENTERING SAID EVAPORATOR BODY THROUGH SAID FEEDING MEANS; TEMPERATURE SENSING MEANS FOR SENSING CHANGES IN TEMPERATURE IN SAID EVAPORATOR BODY; AND CONTROL MEANS RESPONSIVE TO SAID SENSING MEANS FOR VARYING THE TRANSMISSION RATIO OF SAID VALVE AS A FUNCTION OF THE TEMPERATURE CHANGES SENSED BY SAID SENSING MEANS TO INCREASE THE TRANSMISSION RATIO IN RESPONSE TO A TEMPERATURE RISE IN SAID BODY AND TO DECREASE THE TRANSMISSION RATIO IN RESPONSE TO A TEMPERATURE DROP IN SAID BODY. 